Bile salts play a crucial role in the digestion of fats in the pathogenesis of cholesterol gallstones. Their physiological activity derives from the ability to form micelles, small aggregates which can "solubilize" fatty acids, triglycerides, phospholipids, and cholesterol. We propose a spectroscopic and enzymatic study of the structural and dynamic properties of model biliary and fat digestion systems, designed to provide a molecular rationale for the biological function of these micellar aggregates. Our spectroscopic strategy employs a combination of 2H and 1H nuclear magnetic resonance (NMR) techniques. The 2H NMR relaxation experiments will focus on acyl chain dynamics in the solubilized species, while high field 1H NMR spectra will be used to map out site-specific interactions between bile salts and other lipid additives. Both NMR techniques will also be used in complimentary fashion with light-scattering determinations of overall aggregate size and shape. Enzymatic accessibility of bile salt-phospholipid and bile salt-triglyceride substrates will be evaluated for the interfacial enzymes phospholipase-A2 and pancreatic lipase/colipase. The kinetic results will be examined in light of mixed micelle structure and in the context of current theoretical models for the enzymatic activity of surface-active proteins.